L-188 From United States of America, joined Jul 1999, 29795 posts, RR: 58 Posted (9 years 4 months 12 hours ago) and read 2485 times:
Hey guys first posting since I left for dispatch school about 4 weeks ago. Finishes up this week. Anyway a question was asked in class that doesn't really relate to the test, but rather the aerodynamics and performance of the Beech 1900C, which is one of the example aircraft in the class and is the one that we are using as our "Airline" airplane. Usually I can come up with a reasonable explanation for why an airplane does what it does, why it does it ect. but I am stumped for this one.
I refer those of you to the recommended cruise power chart, 1550 RPM, ISA -10C that is figure 25 in the FAA ATP and Dispatcher testing supplement, page 19. The chart show the power and fuel flows in the 1900 at various weights and power settings. For example at 16,000 lbs a 1900 will have a fuel flow of 722, 672, and 648 lbs per hour at 22, 24 and 25 thousand feet respectively. At 10,000 lbs at the same altitudes the fuel flows would be 728,680, and 656 lbs respectively.
All of the fuel flows at both of those rates follow what I have learned in many different A&P and pilot courses, namely that as altitude increases, fuel flow decreases. However what the question was, is "Why doesn't flew flow decrease with decreased weight". For example at 22 thousand feet and 16,000 lbs the fuel flow is 722lbs per hour. At 10,000 lbs it is 728 LBs per hour. Some 6 pounds per hour higher.
This seems to be a paradox, since by the logic of most of us, as weight decreases we should need less power to keep the airplane up. The only thing I could think of was that at the lighter weights, more downforce (lift) was needed at the tail and this created more drag that caused the higher fuel flows. But I have my doubts on the reliablity of this hypothisis.
Any aerodynamic experts or 1900 drivers out there can shed some light into why a 1900C that weighs less burns more gas at the same altitude then a heavier one?
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Flybyguy From United States of America, joined Jun 2004, 1801 posts, RR: 1
Reply 1, posted (9 years 4 months 7 hours ago) and read 2396 times:
Maybe there are some trim usage issues with the Beech 1900, because the increase in fuel burn at 22,000 ft @ 16,000lbs is only slightly less than at 10,000 lbs.
Quoting L-188 (Thread starter): The only thing I could think of was that at the lighter weights, more downforce (lift) was needed at the tail and this created more drag that caused the higher fuel flows.
This can make sense since the only place baggage is loaded on a Beech 1900 is in a large rear hold behind the passenger cabin.
Maybe the difference in weights on your specs. sheet has more to do with difference in baggage loads. It is possible that the Beech 1900 is "nose heavy" to counteract the moments of the baggage hold when a full load is placed. To keep the plane flying straight and level may require some trimming to counteract these pitching forces (i.e moments or torques) and thus leads to some drag over a cleaner configuration achieved when the plane is fully loaded with passengers and baggage.
It is my experience as a passenger, that planes this size without symmetric (i.e. fore and aft of the wing) baggage spaces weight and balance can be a bit tricky and inconvenient, especially when the flight attendant or pilot tells you to change your seat to help balance the plane.
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Lowrider From United States of America, joined Jun 2004, 3220 posts, RR: 10
Reply 4, posted (9 years 4 months ago) and read 2335 times:
Does the question give you a CG location? It is very easy to load the beech to an aft CG. Theoretically, you may have a lower AOA on the wing at 16000 lbs with and aft CG then at 10000lbs with a fwd CG. Just a thought.
Goldenshield From United States of America, joined Jan 2001, 6004 posts, RR: 14
Reply 6, posted (9 years 4 months ago) and read 2331 times:
The questions referenced in the question, BE31-35, do not reference any center of gravity; however, there are a 25 questions in the B-1900 section that do deal with COG. Ten deal with weight and balance. Ten deal with a shift in weight and balance (I hated those the most.) Five deal with COG outside of allowable range.
If I remember correctly, the B-1900 questions are part 135 only.
[Edited 2005-04-26 18:09:04]
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L-188 From United States of America, joined Jul 1999, 29795 posts, RR: 58
Reply 8, posted (9 years 3 months 4 weeks 1 day 19 hours ago) and read 2275 times:
Quoting Flybyguy (Reply 1): To keep the plane flying straight and level may require some trimming to counteract these pitching forces (i.e moments or torques) and thus leads to some drag over a cleaner configuration achieved when the plane is fully loaded with passengers and baggage
See that is exactly what I was thinking when I suggested my original answer to this classmate. But the more I think of it the less sure of that answer I am....There just can't be that much drag caused by trimming a light airplane
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LeanOfPeak From United States of America, joined Oct 2004, 509 posts, RR: 1
Reply 11, posted (9 years 3 months 4 weeks 1 day 10 hours ago) and read 2183 times:
The lighter airplane is flying too fast, too low, and too nose-down.
There is an immensely complicated interrelation of variables that determines an aircraft's aerodynamic efficiency in Steady Level Unaccelerated Flight (SLUF), or cruise, but some of them are required lift (Which, in cruise, equals weight), airspeed, altitude, and (It's not usually looked at this way, as it is more often a function of coefficient of lift, but it can be derived at a particular airspeed and altitude) angle of attack.
By the constraints of the table, you have locked in altitude and, to some extent, airspeed (Because dispatchers don't like to slow the airplane down during the trip). The remaining variable to take lift away from the aircraft (When it is lighter) is angle of attack. Unfortunately for this example, there is an angle of attack for optimal aerodynamic efficiency given values for all the other variables, and on either side of that angle of attack, the lift-to-drag ratio falls off.
Bluntly, you are accepting an aerodynamic penalty for operating at a preferred altitude and airspeed. An aircraft which is operating light of optimal can get closer to an optimal angle of attack by either climbing or slowing down, either of which will comparatively raise the nose once SLUF is reestablished.
You, as an aspiring dispatcher, know why they don't like to slow down.
Most long-range aircraft will do some amount of climbing after the initial cruise segment as they get lighter to regain efficiency, but ATC and operational considerations, aircraft limitations, weather, and cost/benefit for the remaining sector length can lead to suboptimal aircraft operation.
So, the engineering answer, which may be more complicated than they're looking for or something entirely different than what they're looking for, is that the lighter aircraft is flying too low and too fast for optimal aerodynamic efficiency (As this results in an attitude nose-down of optimal).